2 I. Iglesias et al. 1. INTRODUCCIÓN La industria de la cerámica de construcción suele utilizar ensayos tecnológicos para caracterizar mecánicamente las piezas que produce. Son ensayos sobre piezas enteras (ladrillos, tejas, bovedillas, etc.) que reúnen el imprescindible requisito de ser repetitivos y de ser comparables con otros ensayos realizados según el mismo procedimiento. Sin embargo, cuando se trata de estudiar la composición óptima de la mezcla o de comprobar el proceso de fabricación más adecuado, dichos ensayos son incómodos, ya que requieren poner en marcha todo el proceso industrial para obtener únicamente unas pocas probetas de ensayo. Por supuesto, en laboratorio se dispone de extrusoras, hornos, etc., a escala que simulan el proceso de fabricación industrial del material, pero los procedimientos de ensayo usados en laboratorio para obtener propiedades mecánicas no están normalizados y sus resultados son difícilmente comparables con los que se hagan en otro lugar con similar procedimiento. Paradójicamente, la tecnología de otros materiales de construcción, como el hormigón, cuenta con mucha normativa sobre los métodos de ensayo de laboratorio para obtener propiedades mecánicas. Sería, pues, sencillo adaptar algunos de estos procedimientos de ensayo a las características particulares de las cerámicas de construcción. Por este motivo nos propusimos aplicar el ensayo brasileño sobre probetas cilíndricas obtenidas por prensado como método de caracterización mecánica básica de cerámicas de construcción en laboratorio. Este ensayo proporciona la resistencia a tracción siempre que se cumplan una serie de proporciones entre el diámetro de la probeta, el ancho de reparto de la carga y la longitud característica del material (1). La interpretación de los resultados del ensayo y sus condiciones de validez han sido extensamente estudiadas por Rocco, Guinea, Planas y Elices en el caso del hormigón (1-4). Una de sus conclusiones es que se trata de un ensayo que puede mostrar efecto de escala en el caso de que el material sea dúctil y el diámetro de la probeta sea relativamente pequeño (2, 3). También señalan que las condiciones de contorno del ensayo influyen mucho en el resultado. En especial, la relación del ancho de reparto de la carga sobre el diámetro debe mantenerse en valores pequeños, del orden de 1/10 (2, 3). Por último, los mecanismos de rotura son indicativos de la validez del ensayo y pueden estudiarse por medio del patrón de fisuración (4). La propuesta que hacemos en este estudio sigue la metodología expuesta en los artículos citados y tiene en cuenta sus recomendaciones. El procedimiento que adoptamos es el siguiente. Fabricamos una serie de probetas cerámicas por prensado a partir de una mezcla de material arcilloso tomada de una fábrica de tejas curvas 1. INTRODUCTION It is custumary for the construction ceramics industry to use technological tests to characterize the mechanical resistance of the specimens that it makes. These tests are carried out on complete components (bricks, tiles, jack archs, etc.) which fulfil the necessary requirements of being both repetitive and comparable with other tests carried out according to the same procedure. Nevertheless, when it comes to studying the right composition of the mixture or to checking the most appropriate manufacturing process, these tests are inefficient because they need to put into motion the whole industrial process in order to obtain only a few samples. Of course, we can find extrusion presses, ovens, etc. in the laboratory on a scale which simulate the process of fabrication of this material, but the test procedures that are used in the laboratory in order to obtain the mechanical properties are not standardized and their results are very difficult to compare with others carried out elsewhere and through a similar procedure. Paradoxically, there are many Standards on the technology of other construction materials and on the test methods used to obtain mechanical properties. Therefore, it would be quite simple to adapt some of these test procedures to the particular characteristics of construction ceramics. For this reason, we set out to apply the Brazilian test on cylindrical samples obtained by pressing as a method for the basic mechanical characterization of construction ceramics in the laboratory. This test can provide the tensile strength only when certain ratios are fulfilled between the diameter of the sample, the load bearing width and the characteristic length of the material (1). The interpretation of the test results as well as their conditions of validity have been widely studied by Rocco, Guinea, Planas and Elices for the case of concrete (1 4). One of their conclusions is that this test can show a scale effect when the material is ductile and the diameter of the sample is relatively small (2, 3). They also point to the fact that the test boundary conditions exert great influence on the result. Particularly, the ratio of the load bearing width over the diameter must be kept under low values, in the order of 1/10 (2, 3). Lastly, the fracture mechanisms are a sign of the test validity and they can be studied through the crack pattern (4). The proposal that we make in this study follows the methodology used in the articles that have already been cited (1 4) and it takes into account their recommendations. The procedure that we take is the following: we make a series of samples through pressing from a mixture of clay taken from a factory of round tiles 418 Mater. Construcc., Vol. 61, 303, , julio-septiembre ISSN: doi: /mc

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